1. Field of the Invention
The invention relates to a horizontal deflection circuit for a picture display apparatus, comprising:
a horizontal output stage provided with a switching element which is coupled to a horizontal output transformer for generating at least one voltage, and PA1 a drive circuit for generating a drive signal for switching the switching element, and provided with a duty cycle control circuit for modulating a duty cycle, of the drive signal during changes of state of the horizontal deflection circuit.
The invention also relates to a method of horizontally deflecting a cathode ray of a picture display tube, and to a picture display apparatus provided with the horizontal deflection circuit.
2. Description of the Related Art
Such a horizontal deflection circuit is known from German Patent Application DE-A-4021940, corresponding to U.S. Pat. No. 5,381,329. This Application describes a power supply circuit in which a switching element (a transistor in this case) is coupled to a power supply transformer and a horizontal output transformer. Such a power supply circuit, which is known as Wessel circuit, supplies power supply voltages by means of the power supply transformer and a horizontal deflection current, fly-back voltages and/or scan voltages by means of the horizontal output transformer. In normal operation, the power supply circuit, further referred to as combined circuit, generates a drive signal of which one edge is used for fixing a switch-off instant of the switching element. This switch-off instant initiates a horizontal fly-back, and is controlled in normal operation by what is generally referred to as a phi2-control circuit. To this end, the drive circuit compares instants of occurrence of fly-back pulses supplied by the horizontal output stage and related to the horizontal fly-back, on the one hand, with instants of occurrence of the horizontal synchronizing pulses, on the other hand. A possible difference in instants of occurrence is corrected so that the video signal is displayed at the correct horizontal position on the display tube.
During a described change of state from stand-by operation to normal operation, a duty cycle of the drive signal is controlled so as to continuously increase an on-time of the switching element from a small value to a nominal value. With such a variation of the duty cycle, the switching element is protected from a too large dissipation. However, a duty cycle modulation, as used in the combined circuit, does not provide the possibility of having a variation of voltages generated by the combined circuit during the change of state to satisfy various requirements imposed by different components that are coupled to the scan and fly-back voltages.